Semiconductor devices are used in many electronic and other applications. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits.
One type of semiconductor device is a memory device, in which data is typically stored as a logical “1” or “0.” A memory device may be static or dynamic. A dynamic memory device needs to be refreshed to “remember” the data, whereas a static memory device does not need to be refreshed to retain stored data.
One type of static memory device, also referred to in the art as a non-volatile memory (NVM) device, is a floating gate memory device. A floating gate memory device can be either erasable programmable read-only memory (EPROM) or electrically erasable programmable read-only memory (EEPROM). Both these type of floating gate memories rely on a charge stored in the floating gate (or a charge trap layer) by suitable application of a bias to the various terminals of the device. The charge may be stored by a number of mechanisms such as carrier tunneling and/or injection. The charge may be removed either electrically as in EEPROM devices or by an external source such as an ultra violet light. The presence of this charge in the floating gate determines the state of the memory as logical “1” or “0.” Flash EEPROM memories are called such due to their fast program and erase times (as in a lightning flash).
The floating gate devices are usually stacked in large arrays to form memory cells such as flash memory cells. Based on the stacking or layout of the floating gate transistors, flash memory may comprise a NOR, NAND, or an AND memory architecture. As an example, most commercial memory cards such as memory sticks comprise NAND flash memory cells. Flash memory is among the most popular memories available in the market today. The popularity of flash memory arises partly due to its compatibility with existing CMOS process flows. Flash memory is simply a field effect transistor, except it has a poly-silicon floating gate sandwiched between a tunnel oxide and an inter-poly oxide to form a charge storage layer. However, flash memory devices have some disadvantages or bottlenecks that need to be overcome for continued success.
As circuit designs become more complex and increased processing speeds are demanded, it is becoming increasingly more important and necessary to be able to provide larger numbers of transistors on an integrated circuit (IC) chip without increasing the overall size of the IC chip. One technique for increasing circuit density is to scale down the size of each individual MOSFET device on the IC chip. The performance of the MOSFET device is inversely proportional to the gate oxide thickness. Efforts to enhance performance, therefore, have driven gate oxide thicknesses down, for example, below 14 Å. Scaling of the gate oxides to such thicknesses, however, leads to high tunneling currents and consequently poor charge retention.
An alternative approach to extreme scaling of oxide thicknesses is the incorporation of nitrogen into the gate oxide. Addition of nitrogen increases the dielectric constant of the gate dielectric over conventional silicon oxide, thereby reducing the effective oxide thickness while reducing gate leakage current. However, nitrogen also increases traps within the dielectric and results in poor reliability or product lifetime.
Hence, what are needed are dielectrics and means to form dielectrics for non-volatile memories that increase charge retention without a decrease in product lifetime.